1. Field of the Invention
The present invention relates to an alignment method for positioning a mask or a substrate with respect to an exposure apparatus.
2. Related Background
When a semiconductor device or a liquid crystal display is to be manufactured, a fine pattern which is formed on a mask or a reticle (hereinafter called the mask) is projected and exposed onto a photosensitive substrate such as a wafer or a glass with a photoresist coated thereon, by an exposure apparatus.
Such pattern exposure is normally conducted by exposing another pattern on the pattern which has already been formed on the photosensitive substrate in a superposing manner several times repeatedly. Since an accuracy in the superposition of the patterns has a direct effect on the performance of the device to be manufactured, it is required to align the mask or the photosensitive substrate with the main body of the exposure apparatus, or the mask with the photosensitive substrate with high accuracy of 1 .mu.m or less. To this end, alignment marks formed on the mask and the photosensitive substrate are detected by an alignment system so that the positions of the mask and the photosensitive substrate are measured, thereby conducting the mutual positioning. The alignment system is, for example, provided with an alignment microscope and an image pick-up device so that the alignment marks enlarged by the alignment microscope are detected by the image pick-up device and image-processed, thereby detecting the alignment marks. The alignment system normally has a narrow field of view of several hundreds .mu.m or around since an accuracy in measuring the alignment marks is degraded when the field of view is widened.
Incidentally, the mask or the photosensitive substrate (hereinafter simply called the substrate) is first mounted on a predetermined position of a mask stage or a photosensitive substrate stage of the exposure device by a carrying mechanism such as a robot or the like. However, the positioning accuracy (loading accuracy) of the substrate which is carried and mounted onto the exposure apparatus by the carrying mechanism is in a range from several hundreds .mu.m to several mm. Especially, since a glass plate (photosensitive substrate) used in an exposure apparats for liquid crystal which manufactures a liquid crystal display device is of a large size of 500.times.600 mm or around, the above-mentioned positioning accuracy (loading accuracy) becomes more liable to be degraded. Accordingly, the position of the substrate mounted on the mask stage or the substrate stage is detected by use of the alignment system by an operation called a search alignment, so that said substrate is repositioned with the positioning accuracy of several .mu.m or the like.
For the search alignment, it is necessary to measure an alignment mark which exists somewhere on the substrate in a comparatively wide area, compared with the field of view of the alignment system, with accuracy of several .mu.m or around, so that said area is divided into a several blocks in order to conduct the alignment.
Description will be made, with reference to FIGS. 5A to 5D, on an operation for measuring the position of an alignment mark (substrate) with the loading accuracy of .+-.0.5 mm (1 mm in all directions) by use of the alignment system which has the field of view (processing range) of 0.5 mm in all directions.
Since the position of an alignment mark AM in a substrate loading range of 1.times.mm indicated by the broken line in each of FIGS. 5A to 5D, is not known, it is assumed that the field of view of the alignment system is first set at the position indicated by the solid line in FIG. 5A. In this case, if the alignment mark does not exist in the field of view of the alignment system, the substrate and the alignment system are relatively moved by, for example, moving the substrate stage by a distance substantially equivalent to the size of said field of view, as indicated by the arrow in FIG. 5B, so as to conduct the search and the measurement of the alignment mark. On this occasion, an amount of movement of the substrate is monitored by a position monitoring means such as a laser interferometer or the like, to be used as an offset for the measurement of the alignment mark. If the alignment mark AM is not found within the field of view of the alignment system even in a state shown in FIG. 5B, the substrate is repeatedly moved, as indicated by the arrow in FIG. 5C and FIG. 5D, until the alignment mark comes into the field of view. When the alignment mark comes into the field of view, the image of said alignment mark is processed and the position of the alignment mark in the field of view of the alignment system is measured. Then, the amount of movement of the substrate up to that time is added as the offset to a measurement value obtained by said position measurement, whereby the position of the alignment mark AM, i.e. the position of the substrate is detected.
Since it is necessary to cover a wide area by use of an alignment system having a narrow field of view according to a conventional alignment method as described above, the search and the position measurement of an alignment mark is conducted by dividing a range in which the alignment mark is expected to exist into a plurality of areas. Therefore, a time required for the alignment (measurement) is increased.
Also, when the field of view of the alignment system and the substrate are relatively moved, a measuring device for monitoring an amount of such movement is required. Also, an accuracy in such measurement is required to be as high as that required for the alignment.